This invention relates to an assembly for cryogen gas venting for a superconducting magnet, particularly suitable for connecting the cryostat vent to the atmospheric vent for the superconducting magnet in the event of undesirable and dangerous high cryogen gas pressure buildup.
As is well known, a magnet can be made superconductive by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
In the event of an undesired magnet quench or reversion to a non-superconductive state rapid potentially dangerous cryogen gas high pressure buildup in the cryostat requires pressure relief through rapid venting of the cryogen gas to the atmosphere outside the superconducting magnet. A replaceable burst disk assembly provided in the vent assembly ruptures at a predetermined pressure to open the vent to the atmosphere.
However, it has proven difficult as a practical matter to properly seal the vent assembly, yet provide for ready replacement of the burst disk and resealing of the vent assembly after a high pressure rupture.
One problem encountered is the inability of gaskets such as neoprene or silicone to withstand multiple extreme temperature cycles encountered as a result of periodically placing the MRI magnet into superconducting operation and the alternate cryogen refill and venting. Over time this has led to cracks in the gaskets and leakage of helium gas during operation. In addition, some materials have exhibited creep which also leads to helium gas leakage.
The problem of helium gas leakage is most significant in superconducting magnets which recondense the helium gas back to liquid helium and which are often referred to as zero boiloff (ZBO) magnets designed to minimize the difficulties encountered in shipping and storing the necessary reserve supply of liquid helium at cryogenic temperatures and the related problems of periodically transferring a portion of the liquid helium in the storage reservoir to the liquid helium supply in the MRI superconducting magnet.
A helium gas leak at the burst disk assembly will lead to the need to continuously add or replenish the liquid helium defeating the goal and advantages possible with a ZBO superconducting magnet.